This invention relates to cooling systems for submersible propulsor units, and is particularly concerned with a closed system and method for cooling a propulsor unit of the type having a housing that contains a submersible stator and rotor.
Electric motor type propulsor units are known in the prior art. While such propulsor units may be used as primary propulsors for either surface ships or submarines, they find their primary application as secondary drive units for such vehicles. Originally, such secondary propulsor units were intended to be used as a backup drive in cases where the main propulsor became inoperative so that the ship or submarine could "limp" back into a port where the necessary repairs of the main propulsor unit could be carried out. More frequently, however, such secondary propulsor units are used to steer such craft during docking maneuvers since the electric motors that power them are capable of producing the low amounts of highly controlled thrust that are necessary for such maneuvers.
Prior art secondary propulsor unit are typically formed from an electric motor having a "canned" stator stationarily mounted within a torpedo-like housing. A shaft is journalled on radial bearings disposed on either end of the housing, and a canned rotor is mounted around the shaft in concentric alignment wit the stator. One of the ends of the shaft extends through the housing and has a propeller mounted thereon, while a thrust bearing is provided between the housing and the other end of the shaft for handling the thrust load generated by the propeller as it screws through the water. Because the "canned" stator and rotor are water tight, ambient sea water may flow through various openings in the housing to both dissipate the heat generated by the electric motor, as well as to cool and lubricate the thrust bearings and radial bearings disposed within the housing. One of the advantages of this prior art design is that there is no need for the relatively complex, expensive and delicate pressure-bearing shaft seals required for primary propulsor units which are necessary to prevent sea water from entering the engine room of the vessel.
Unfortunately, while there are advantages in allowing the ambient water to circulate within the housing of such propulsors, the applicants have also noted some disadvantages to this design. For example, the dissolved salts and other minerals present in ambient sea water have been found to be highly corrosive to the metallic surfaces within these housings, despite the use of anti-corrosive materials. Such corrosion typically necessitates a complete overhaul of such propulsor units every two to three years on the average, which is both time consuming and expensive, as the entire vehicle must be dry docked or at least put out of operation until such overhauls are complete. A second problem associated with the design of such prior art propulsor units is caused by the particulate debris entrained in the ambient water which is ultimately carried into the interior of the housing. Such particulate debris not only exacerbates abrasive wear between bearing surfaces; it can also collect at various locations within the interior of the housing to a point where the free circulation of water through the housing is obstructed. This last problem is rendered more acute by the increased use of such propulsors to perform docking maneuvers in relatively shallow waters where the propulsors tend to stir up silt and other types of fine particulate debris into the ambient water.
Clearly, there is a need for a secondary propulsor unit which maintains all of the advantages associated with prior art propulsor units whose housings require no expensive and delicate pressure-bearing shaft seals, but which does not allow the dissolved salts and entrained particulate debris to corrode, abrade or accumulate within the interior of the propulsor housing.